Azabora[5]helicene Charge‐Transfer Dyes Show Efficient and Spectrally Variable Circularly Polarized Luminescence

Abstract

Three helicenes based on a borylated arylisoquinoline skeleton have been prepared in their enantiopure forms and characterized with respect to their photophysical properties, including the use of chiroptical spectroscopies. The dyes show varying charge-transfer characteristics and efficient emission (quantum yields between 0.13 and 0.30, in toluene), which is governed by the electron-donor substitution (p-MeO-phenyl, p-Me N-phenyl) at the helicene. Marked differences in the emission wavelength and Stokes shift are observed, with the dimethylamino-substituted derivative emitting most red-shifted (maximum at ca. 590 nm) and displaying the highest Stokes shift (ca. 6000 cm ) in toluene. The helicenes show electronic circular dichroism (ECD) and significant circularly polarized luminescence (CPL) with dissymmetry factors of up to 3.5×10 . The sign of the ECD band corresponding to the first transition and of the CPL spectrum depend sensibly on the electron-donor substitution.

Quantifying the Overall Efficiency of Circularly Polarized Emitters

“…Given both the large number of reported CPL‐active [ n ]helicenes and also their quite diverse structural motifs (Scheme 4), we shall distinguish and base the discussion of the surveyed compounds on five main classes: 1) purely monomeric organic [ n ]helicenes (in turn divided into 6 subclasses according to n, with n being the number of aromatic units forming the helicene scaffold); [38–65] 2) dimeric systems; [48, 50, 66–68] 3) S‐shaped systems; [69, 70] 4) systems with semimetals (B and Si) in the [ n ]helicenes core or directly linked to it; [71–76] and 5) cationic systems [77–80] (Scheme 4 and Table 4).…”

“…The compounds with Si and B embedded in the helicene scaffold or directly bound to one of the scaffold aromatic units ( Hel‐60 – Hel‐71 ) present extinction coefficients in the range of 1000–34000 m −1 cm −1 and their emission fall in the blue‐yellow region (430–586 nm) [71–76] . The quantum yields are quite dispersed, going from 0.07 (in the case of Hel‐63 and Hel‐64 [72] ) to 0.65 ( Hel‐70 [75] ), and the g lum factors are generally low (in the order of 10 −4 –10 −3 ).…”

Charging OBO‐Fused Double [5]Helicene with Electrons

“…[11] As ar esult, B-fused helicenes are still very rare and they are mostly stabilized by adjacent nitrogen (N) or oxygen (O) atoms. [12] Thec oncomitant incorporation of Band N/O atoms enhances the stability, but compromises the electron-accepting properties.T oo ur knowledge,t he electron-accepting behavior of B-fused helicenes or multiple helicenes has never been revealed. Recently,w eh ave developed an ew type of OBO-fused double helicenes with excellent stability.…”

Azahelicene‐Fused BODIPY Analogues Showing Circularly Polarized Luminescence

“…However, most helicenes reported so far showed CPL in a blue region of the spectrum partly because functionalization of helicenes to tune the CPL wavelength is difficult. Some nitrogen‐containing helicenes show CPL in the visible light region through coordination, protonation, or donor–acceptor interactions …”